1. Field of the Invention
The present invention is directed to a semiconductor chip carrier assembly and a process for making that assembly. More particularly, the present invention is directed to a chip carrier assembly utilizing a microporous film adhesive to bond the flexible circuit and the stiffener members.
2. Field of the Prior Art
It is well known in the art to electrically connect a semiconductor chip to a circuit board by employing a carrier structure for the semiconductor chip. One well accepted chip carrier assembly design for providing this electrical connection is a tape ball grid array type assembly (TBGA). A TBGA assembly is electrically connected to a circuit board by means of solder balls. This electrical connection is accomplished by connecting the solder balls of the chip assembly to electrical connecting pads on the circuit board.
The aforementioned assembly requires the inclusion of a metal stiffener to provide adequate structural support for the very thin polyimide tape which constitutes the flexible circuit substrate.
A chip carrier assembly stiffener is bonded to the polyimide tape substrate by means of an adhesive. One commonly employed adhesive is fiberglass reinforced pressure sensitive acrylic that acts not only as an adhesive but also as an electric insulating barrier to prevent solder rivets from electrically communicating with the stiffener. Unfortunately, acrylic film adhesives, although somewhat conformable, do not entirely conform around the protruding solder rivets. As such, the use of acrylic film adhesives oftentimes result in the formation of pockets of air around the base of the rivets. Although this film adhesive is designed to overcome this problem, insofar as it is characterized by an embossed pattern to channel out air during assembly, air may still be entrapped between the flexible circuit and the adhesive.
The entrapment of air between the flexible circuit and the adhesive serves as a channel which permits the introduction of moisture. As those skilled in the art are aware, moisture causes corrosion failures and/or delamination during solder reflow. To avoid this, semiconductor chip assemblies that employ pressure sensitive acrylic film adhesives include the further processing step, prior to solder reflow, of baking the assembly.
There are additional detrimental aspects relating to the use of pressure sensitive film acrylic adhesives in chip carrier assemblies. Upon curing of the acrylic based adhesive, benzene vapor is produced as a by-product. This outgassing of benzene, a well-established carcinogen, represents an important health hazard.
Another adverse feature of commonly employed pressure sensitive film acrylic adhesives is the inclusion therein of metal oxide particles. The presence of metal oxide particles in acrylic adhesives represents a reliability hazard insofar as it can potentially lead to shorting out of the assembly.
Yet another detrimental result of employing pressure sensitive acrylic film adhesives is their tendency to oxidize upon exposure to long term temperature aging. Exposure of the assembly to this high temperature for this long duration leads to oxidation problems and has a long term detrimental effect upon the chip assembly.
U.S. Pat. No. 5,512,360 describes a prepreg adhesive composite formed from a porous polymeric substrate providing good adhesion, low dielectric constant and low modulus. The composite comprises at least one layer of an expanded polytetrafluoroethylene having an initial void volume of at least 50% to 95% and containing between 15% and 40% by weight, based on the total weight of the composite, of adhesive to form an uncured prepreg that eliminates or substantially reduces adhesive on the exterior, outer surfaces of the expanded polymeric structure. The composite of this patent is employed to make printed circuit boards as well as microwave substrates.
A new semiconductor chip carrier assembly has now been developed which includes an adhesive to bond the flexible substrate to the stiffener which overcomes the problems associated with adhesives employed in the prior art. Although the adhesive of the present invention is a solid it is highly conformable around solder rivet heads and circuit lines included on the flexible substrate. Furthermore, the film adhesive is porous so that air can escape from between the flexible substrate and the adhesive, eliminating air therebetween.
In addition, the adhesive has a low modulus of elasticity which, when combined with its high degree of conformability, results in the absence of mechanical strains. Upon curing, no toxic and/or carcinogenic vapor, such as benzene, is exuded. Moreover, the problem associated with oxidation, resulting from long term temperature aging, is considerably reduced due to the low concentration of ionic components in the adhesive. Finally, another problem usually associated with long term curing, its considerable duration, is reduced because of the shorter curing period associated with the curable resin employed in the adhesive composition of the present invention.
In accordance with the present invention, a semiconductor chip carrier assembly is provided. The assembly includes a flexible substrate having a first and a second surface, the first surface including metallicized paths to provide electrical conductivity therethrough. The first surface is also provided with electrical conductive rivets. The second surface of the flexible substrate includes electrically conductive members for electrical communication with a circuit board. The assembly further includes a semiconductor chip in electrical communication with the flexible substrate. A stiffener is included to support the flexible substrate. The stiffener, having a first and a second surface, is disposed such that the first surface of the stiffener is adjacent the surface of the flexible substrate. An adhesive comprising a microporous film laden with a curable thermosetting resin is disposed between the first surface of the stiffener and the first surface of the flexible substrate. A cover plate, adhesively bonded to the semiconductor chip and to the second surface of the stiffener, is also included.
In further accordance with the present invention, a process for making the aforementioned semiconductor chip carrier assembly is disclosed. In this process a flexible substrate is introduced into a vacuum fixture, said fixture having openings to accommodate irregularities on the bottom surface of the substrate. A vacuum is applied to hold said flexible substrate flat on said vacuum fixture. An adhesive composition comprising a microporous film substrate and a curable adhesive is placed over the flexible substrate. A stiffener is disposed over the adhesive composition. The assembly is bonded by curing the curable adhesive by subjecting the assembly to a temperature in the range of between about 140xc2x0 C. and about 200xc2x0 C. and a pressure of between about 50 psi and about 1000 psi.